When using conventional fluid flow control devices near minimum flow or shutoff, a number of factors including: duct configuration, turbulence, leakage, etc., can cause the internal mechanism to become unstable. This can result in inaccurate fluid flow through the device.
In some instances, the rate of fluid flow through the device can change drastically as the regulator closes the valve. Additionally, the internal mechanism can produce undesirable sounds and flexing of the duct which could result in damage to the valve and/or duct that would result in leakage or incorrect flows.